The role of the macrophage as effector cells in tumor immunity is to be studied in the light of the functional heterogeneity of these cells. Subpopulation of rat immune and non-immune peritoneal exudates, obtained by density gradient centrifugation, will be examined for their cytotoxic and cytostatic activity against a Gross lymphoma (C58NT)D, which is maintained and grown in tissue cultures. Allogeneic and Xenogeneic tumor lines from mice and humans will also be used as target cells in the study of nonspecific activity of peritoneal macrophages. Specific activation of macrophages by immune lymphocytes will be performed by co-culturing immune lymphocytes or supernatants for such cultured cells with peritoneal exudate macrophages or subclasses of macrophages from non-immune rats. These specifically armed macrophages will be compared with peritoneal exudate macrophages that have been "activated" in vitro or in vivo with various agents. In addition to investigating cell-mediated destruction of tumor cells by peritoneal exudate macrophages, other sources of macrophages from spleen and lungs will be assayed. Procedures employing Cr51 release and tumor cells enumeration will be used to assay cytotoxicity and cytostasis. The role of macrophages as suppressor cells in tumor immunity will also be investigated by measuring their ability to decrease the formation of a secondary cell-mediated immune response by immune spleen cells in vitro.